The present invention relates to a method and apparatus for manufacturing tubular structures via electrospinning and, more particularly, to a method and apparatus for manufacturing a polymer fiber tubular structure having improved kinking resistance. The present invention further relates to tubular structures having improved kinking resistance.
In many medical and industrial applications, tubular structures made from polymer fibers are used as, e.g., vascular prostheses, shunts and the like. Production of polymer fiber tubular structures is particularly difficult when such tubular structures are required to have radial tensile strength sufficient to resist tearing and collapse in response to a pulsating pressure while at the same time maintain several elastic properties, such as the ability to bend without breaking and without kinking, in order to allow conformation to a complex geometry.
When an elastic tubular product bends, it experience a finite force onto a small surface area, hence the stress concentration at the bending point is high. Consequently, the tubular product is kinked, i.e., it either undergoes destruction, or bends with inner lumen collapse.
A typical method known in the art to prevent such a collapse is to support the surfaces of the tubular product by rigid circular members so that the product is made of alternating elastic and rigid longitudinal sections. Upon axial deformations, the elastic members can freely operate by tension-compression within the limits admissible by the agent elastic properties, while at the same time, development of radial deformations is limited by the presence of the rigid elements.
Radial support of tubular product can be done in more than one way. For example, tube corrugation provides alternating sections with differing diameter but permanent wall thickness. In this case, required rigidity is achieved at the expense of a plurality of wall members oriented at an angle which is close to 90° relative to the tube central axis. Another method is to reinforce an inner or outer wall of an elastic tube, by a rigid spiral pattern made of steel wire or polymer thread of an appropriate diameter. This type of structure can be also found in physiological systems such as the tracheal and the bronchial of the respiratory system, were rigid cartilage-tissue rings are interconnected by the elastic connective tissue.
In the vascular system, blood vessels possess integrity of unique biomechanical properties. Of particular importance is the resistance of the vessel to inner lumen collapse upon sharp “corners”, which ensures normal blood supply.
Production of tubular fibrous products, including artificial blood vessels, is described in various patents inter alia using the technique of electrospinning of liquefied polymer, so that tubular products comprising polymer fibers are obtained. Electrospinning is a method for the manufacture of ultra-thin synthetic fibers, which reduces the number of technological operations and increases the stability of properties of the product being manufactured.
The process of electrospinning creates a fine stream or jet of liquid that upon proper evaporation of a solvent or liquid to solid transition state yield a non-woven structure. The fine stream of liquid is produced by pulling a small amount of polymer solution through space via electrical forces. More particularly, the electrospinning process involves the subjection of a liquefied polymer substance into an electric field, whereby the liquid is caused to produce fibers that are drawn by electric forces to an electrode, and are, in addition, subjected to a hardening procedure. In the case of liquid which is normally solid at room temperature, the hardening procedure may be mere cooling; however other procedures such as chemical hardening (polymerization) or evaporation of solvent may also be employed. The produced fibers are collected on a suitably located sedimentation device and subsequently stripped of it.
Artificial vessels made by electrospinning have a number of vital characteristics, including the unique fiber microstructure, in many ways similar to that of the natural muscular tissue, high radial compliance and good endothelization ability. However, an artificial vessel fabricated using conventional electrospinning does not withstand kinking, and further reinforcement of the final product is necessary.
The inner surface of blood vessel prosthesis must be completely smooth and even so as to prevent turbulence during blood flow and related thrombogenesis. This feature prevents the employment of tube corrugation, since such structure affects the blood flow and may cause thrombogenesis. In addition, the vessel rigid members must ensure radial compliance and, if possible, have fiber structure and porosity similar to that of the basic material of the prosthesis wall. Still in addition, the rigid members should under no conditions be separated from the elastic portions of the prosthesis. On the other hand, in the vascular system, application of various adhesives is highly undesirable. Hence, the above mentioned techniques, to prevent collapse of the vessel lumen are inapplicable.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and apparatus for manufacturing tubular structures, and particularly vascular prostheses, devoid of the above limitations.